This application is to continue our studies of aspirin prophylaxis against postoperative thromboembolic disease (TED). We established that 1.2 grams of aspirin daily, started preoperatively, provides safe and effective reduction in deep venous thrombosis (DVT) in men following total hip replacement. However, women are not protected. These two observations are of major importance in the pharmacology of antiplatelet agents and the reduction of venous TED. Studies by others show that sulfinpyrazone prevents thrombi in arterio-venous shunts in men, not women, and that aspirin reduces death and recurrent strokes in men but not in women. The fact that aspirin provides simple, safe, inexpensive and effective prophylaxis against DVT in one group of males at high risk requires further study in other groups. That 1.2 grams of aspirin does not reduce DVT in women also requires further study, specifically to determine if this absence of effect can be overcome. Aspirin prophylaxis will be assessed in four high-risk groups to a) quantify the role of higher doses in providing protection, b) study further the sex-related differential response, c) evaluate whether the difference can be offset by higher doses given more frequently, d) establish basic data on the prevalence of DVT in the control population for each group and e) assess aspirin's role in these other groups. We will also study pulmonary emboli. Despite the failure of 1.2 grams of aspirin to reduce DVT in women, among 528 total hip replacement patients protected with aspirin, half of whom were women, the prevalence of pulmonary emboli was the same in men and women and no fatal pulmonary emboli occurred. Zekert also found that aspirin protected women against fatal pulmonary emboli after hip fractures. Does the aspirin have an effect on the prevalence of pulmonary emboli independent of its role of DVT or is there a beneficial effect on the cardiopulmonary impact of emboli if they do occur? Pulmonary emboli will be quantified by preoperative and serial postoperative radioactive pulmonary scanning, not with conventional V/Q scans, but using the more accurate and more sensitive new technique of CO2 scanning using 15O-labeled carbon dioxide, confirmed when indicated by pulmonary angiography.